The sinuous stream of the Mattes follows a spectacular narrow pass between
Mount Alaric and the hill of La Coque, called the Gorges of Congoust (latin co-angustum: narrow pass).
The landscape presents contrasting vegetation: very dry on the South slope of Mount Alaric, much more densely forested on the slope of the hill of La Coque which is covered with green oak bushes.
The route is framed by limestone cliffs and massive collapsed rocks; waterfalls and natural water basins provide rhythm and grace to the stream.
The very picturesque ruins of the former priory of Les Illes seem to still be guarding the entrance of the gorges. Walking path GR36 crosses the gorges of the Congoust stream. On one side it is possible to reach Lagrasse via Ribaute. On the other side, it goes to Montlaur via the ruins of Saint-Michel de Nahuze.
Next to this walking path on the side of the road, there is also a cave, already explored by speleologists. During heavy rain, this large cave collects water from the hill which overflows in the stream of Mattes, which makes its exploration dangerous.